Iron complexes spin state interconversions

The most significant results obtained for complexes of iron(II) are collected in Table 3. The data derive from laser Raman temperature-jump measurements, ultrasonic relaxation, and the application of the photoperturbation technique. Where the results of two or three methods are available, a gratifying agreement is found. The rate constants span the narrow range between 4 x 10 and 2 X 10 s which shows that the spin-state interconversion process for iron(II) complexes is less rapid than for complexes of iron(III) and cobalt(II). 

Table 3. Rate constants and activation parameters for the LS HS and HS LS unimolecular spin-state interconversions in iron(II) complexes...

If the spin state interconversion is faster than the excited nuclear state lifetime, that is x 10 7 second, then the observed spectrum is an average of the spectra of the two spin states. Until recently this condition had been observed only for iron(III) complexes with thiocar-bamate or selenocarbamate ligands—ferric dithiocarbamates (119), monothiocarbamates (98), or diselenocarbamates (42). Since 1982, however, there have been a number of reports of other iron(III) complexes which also display an averaged Mossbauer spectrum (56, 57, 108 111, 124, 153, 155). 

Fe(benzac2trien)]+ (20). These observations, together with the short relaxation times presented in Table IV, indicate that spin state interconversions in iron(III) complexes are slightly more rapid than those observed in iron(II) complexes. There are only a limited number of results available at present, however, and these may not be representative. 

The rate of the spin state change for the octahedral cobalt(II) complexes is expected to be faster than that observed for the iron(II) and iron(III) complexes. In the cobalt(II) case the spin state change involves only one electron, that is AS = 1. The 2E and 4T states are directly mixed by spin-orbit coupling (10, 163). The spin state transition should be adiabatic, with k = 1, without any spin-forbidden barrier. Furthermore, the coordination sphere reorganization involves a change in bond length of 21 pm along only two bonds, instead of all six bonds as in iron complexes. Both of these factors lead to the prediction of rapid spin state interconversion. 

There is some evidence that the strength of intermolecular forces determines the degree of cooperativity and the rate of spin state interconversion in the lattice (154,155). This is a reasonable hypothesis, for it assumes a continuum of behavior, from very weak interactions, which reflect intramolecular properties, to strong intermolecular forces, which cause cooperative phase transitions and abrupt spin state changes. Neutral complexes with a molecular lattice and little or no hydrogen bonding between the molecules, such as some iron(III)... 

The EPR spectrum of a spin-equilibrium complex can be used to establish a lower limit to the spin state lifetimes of the order of 10 10 second. In an important paper in 1976, Hall and Hendrickson reported observation of EPR signals for both the high-spin and the low-spin isomers of iron(III) dithiocarbamate complexes at 4 12 K as powders, glasses, and doped solids (71). This resolved the question whether these complexes possess distinct high-spin and low-spin states. It also sets a lower limit on their interconversion lifetimes. Similarly, the observation of signals for both the high-spin and low-spin states of [Co(terpy)22+] (97) leads to the same conclusions about this complex. In both cases the interconversion rates in solution have proved too fast to measure, with lifetimes of less than 10-9 second indicated. The solution measurements were undertaken, of course, at room temperature and the EPR measurements at close to 4 K. Significant differences in the rates of solid and solutions at room temperature are still possible. 

It cannot be excluded that for first-row transition metal complexes, in addition to the above factors, the central atom spin state also plays a significant role in the stability of individual electronic and/or spin isomers, and in their ability to undergo photochemical interconversions. For some iron complexes such spin-state photoisomerizations are well-known [1, 121]. 

An opposite situation is found for the catalytic N O decomposition over Fe/ZSM-5 [46]. Although mononuclear Fe sites are highly active for the dissociation of the first N O molecule, further catalytic reaction is hampered in this case by a very high barrier for the activation of the second N O molecule necessary for the formation and completion of the catalytic cycle. The binuclear [Fe(p-0)2Fe] + and [Fe(p-0)Fe] + complexes are much more catalytically active. Each iron center in such complexes acts as an independent Np dissociation center. This allows the formation of two proximate labile extra-framework O species, which easily recombine to form molecular O. The reaction mechanism in this case is highly complex (Fig. 9). It involves multiple changes in the spin-state of the iron complexes as well as the interconversion between the [Fe(p-0)2Fe] and [Fe(p-0)Fe] active complexes in the course of the catalytic process. The identification of the preferred reaction channel based only on the results of DFT calculations is not possible. 

The water complexes are high spin, whereas the cyanide and phen complexes are low spin. In the case of the cyanide and phen complexes, the interconversion of the 2g iron(ii) and /2g iron(iii) states simply involves the loss or gain of an electron from the 2g level. Since these are the orbitals oriented between the ligand donor atoms. 

T2(Oh) 1A1(Oh) spin equilibrium in iron(II) complexes based on the hy-drotris(l-pyrazolyl)borate ligand, was established for [Fe(HB(me-pz)3)2] (for abbreviations see Sect. 8.1) in the solid state 172>173 and for [Fe(HB(pz)3)2] in solution174 . Sutin et al.17S studied the dynamics of the spin interconversion in CH2C12/CH30H solutions with the laser Raman temperature-jump technique between 0 and 25 °C. The relaxation was observed to be first order with a lifetime of 32 10 ns, independent of temperature and concentration over the range studied. The ki and k j values for the process... 

---